The present invention relates to a technique for fabricating fiducials in a buried heterostructure edge emitting laser for alignment of the device in a passive manner.
The present invention is related to U.S. Pat. No. 5,981,975 to Imhoff, filed Feb. 27, 1998 as well as to U.S. patent application Ser. No. 60/079,910 filed on Mar. 30, 1998, the disclosures of which are specifically incorporated herein by reference. Light emitting devices often utilize double heterostructures or multi-quantum well structures in which an active region of a III-V semiconductor is sandwiched between two oppositely doped III-IV compounds. By choosing appropriate materials for the outer layers, the band gaps are made to be larger than that of the active layer. This procedure, well known to one of ordinary skill in the art, produces a device that permits light emission due to recombination in the active region, but prevents the flow of electrons or holes between the active layer and the higher band gap sandwiching layers due to the differences between the conduction band energies and the valence band energies, respectively. Light emitting devices can be fabricated to emit from the edge of the active layer, or from the surface. Typically, a first layer of material, the substrate, is n-type indium phosphide (InP) with an n-type buffer layer disposed thereon. This buffer layer again is preferably InP. The active layer is typically indium gallium arsenide phosphide (InGaAsP) with a p-type cladding layer of InP disposed thereon. One potential pitfall of double heterostructure lasers is often a lack of means for confining the current and the radiation emission in the lateral direction. The result is that a typical broad area laser can support more than one transverse mode, resulting in unacceptable mode hopping as well as spatial and temporal instabilities. To overcome these problems, modern semiconductor lasers employ some form of transverse optical and carrier confinement. A typical structure to effect lateral confinement is the buried heterostructure laser. The buffer, active and cladding layers are disposed on the substrate by epitaxial techniques. The structure is then etched through a mask down to the substrate level leaving a relatively narrow (roughly on the order of several microns) rectangular mesa composed of the original layers. A burying layer is then regrown on either side of the mesa resulting in the buried heterostructure device. The important feature of a buried heterostructure laser is that the active layer is surrounded on all sides by a lower index material so that from an electromagnetic perspective the structure is that of a rectangular dielectric waveguide. The lateral and transverse dimensions of the active region and the index discontinuities are chosen so that only the lowest order transverse mode can propagate in the waveguide. Another very important feature of the structure and that which is required to effect lasing is the confinement of injected carriers at the boundaries of the active region due to the energy band discontinuities at the interface of the active region and the InP layers. These act as potential barriers inhibiting carrier escape out of the active region.
One area of optoelectronics which has seen a great deal of activity in the recent past is in the area of passive alignment. Silicon waferboard, which utilizes the crystalline properties of silicon for aligning optical fibers, as well as passive and active optical devices, has gained a great deal of acceptance. One technique for aligning an optoelectronic device to an optical fiber and other passive and/or active elements is the use of an alignment pedestal for lateral planar registration and standoffs for height registration. By virtue of the sub-micron accuracy of photolithography used to define and align these pedestals and standoff features, the application of this approach has proven to be a viable alignment alternative. By effecting alignment in a passive manner, the labor input into the finished product can be reduced, resulting in lower cost of the final product.
One example of such an alignment scheme can be found in U.S. Pat. No. 5,163,108 to Armiento, et al., the disclosure of which is specifically incorporated herein by reference. The reference to Armiento, et al. makes use of an alignment notch on the active device which is designed to mate with alignment pedestals and standoffs on the silicon waferboard. This particular structure is used for aligning an optical fiber array to an array of light emitting devices.
FIG. 1 is a perspective view of a laser array die 102 which is to be mounted on a silicon substrate 100 such that the active region 106 of the laser die 102 accurately aligns with a fiber to be placed in a v-groove 105 on the silicon substrate 100. As shown, the die 102 has a notch 101 that has been etched therein to be an accurately controlled distance from the laser active region 106. Further, pedestals 103, 104, 108, and 109 have been fabricated on the substrate at predetermined locations to serve as mechanical fiducials for the laser die 102, i.e., the laser will be aligned by virtue of contact with the fiducial. In particular, the laser die is placed on the silicon substrate 100 generally in the vicinity of fiducials 103, 104, 108, and 109 so that the active region 106 roughly aligns with the v-groove 105. The laser die 102 is then pushed in the z direction so that the front surface 107 of the die 102 abuts mechanical fiducials 108 and 109 and in the x direction so that the surface 112 of notch 101 abuts the surface 113 of mechanical fiducial 103, thereby precisely aligning the laser die 102 on the silicon in the x and z directions in a position dictated by the placement of the mechanical fiducials 103, 104, 109, and 110, (and notch 101).
Unfortunately, one problem with structures like the one shown in the reference to Armiento, et al., is that it pertains only to ridge laser structures. This is because, in a ridge waveguide laser structure, the patterning photolithography step that defines the active waveguide is simultaneously used to define the alignment notch in the same mask level, resulting in an alignment of the notch and active waveguide that is limited only by the variations in the photolithography mask. However, it is advantageous from a performance standpoint to be able to utilize lasers and other active devices that incorporate a regrowth step, such as the buried heterostructure laser described above. For this class of devices, the subsequent regrowth step(s), bury the active waveguide mesa and, hence, also the notch. Accordingly, fabrication is complicated because the alignment notch must be made after the regrowth since the notch patterning step must occur in a photolithography step subsequent to the one in which the active waveguide is defined. Moreover, a notch patterning step on the regrown surface of the wafer is difficult because the mesa is not a visible re-alignment feature using the conventional technique of optical alignment methods. Even further, creating the notch using a different photolithography step and mask than was used to create the mesa increases the potential misalignment between the mesa and the notch. Particularly, in such situations, the tolerances of the masks are essentially cumulative. Further, additional error is introduced by misalignment of the masks to one another.
Another known scheme for passively aligning an optoelectronic device to an optical fiber on a silicon waferboard is the use of visual fiducials and an optical detection system. In this technique, visible markings (the fiducials) are made on the surfaces of the optoelectronic device and mating markings are made on the silicon waferboard. The visual fiducials usually are made by etching through at least the outermost layer of the optoelectronic device and the silicon waferboard to leave an aperture that can be detected by an optical detection system. The fiducials are placed on the optoelectronic device and the silicon waferboard in a pattern so that when the optoelectronic device is positioned on the silicon waferboard so that the fiducials on the optoelectronic device perfectly overlay the mating fiducials on the silicon waferboard, the two are properly aligned.
An optical detection system detects the visual fiducials on the waferboard and optoelectronic device and then controls stepper motors or equivalent means that align the optoelectronic device with the waferboard so is that the fiducials properly mate with each other, thereby resulting in proper alignment in at least two dimensions of the optoelectronic device on the silicon waferboard. The fiducial marks should be two dimensional, such as x""s or squares so as to provide visual reference cues in at least two orthogonal directions. Further, typically two or more separate fiducial marks that are spaced from each other are utilized on each of the optoelectronic device and the waferboard. The use of two or more spaced visual fiducials provide for greater accuracy in alignment, particularly angular rotation about the y axis. Particularly, the more individual fiducial marks and/or the further apart they are from each other, that smaller will be any angular rotation errors due to tolerance limits and the like.
FIGS. 2A through 2C illustrate the concept of visual fiducials. FIG. 2A is a plan view of a silicon waferboard adapted to mate a semiconductor laser to a fiber in a v-groove of a silicon waferboard. FIG. 2B is a close up of the portion of the waferboard surrounding the laser pad and FIG. 2C is a plan view of the laser die. The waferboard 150 includes a laser bond pad 152 on which the semiconductor laser 168 is to be mounted and a v-groove 154 within which the fiber is to be placed. Also shown for sake of completeness are a solder metalization 156 for a monitor pin behind the laser and a wire bond pad 158 for electrically coupling the laser to circuitry (not shown) on or off the silicon waferboard 150. Visual fiducials in the form of squares are shown at 160 and 162 on opposite sides of the laser pad 152. These squares actually comprise holes etched through the various layers of the silicon waferboard.
On laser die 168, strip 170 running longitudinally in the center of the die is the active region of the laser. Mating fiducial marks 172 and 174 are etched in at least the outermost layer of the semiconductor laser die. Visual fiducials 172 and 174 are shaped and positioned to exactly mate with visual fiducials 160 and 162 on the semiconductor die. All four fiducials are positioned such that, when fiducial 172 exactly overlays fiducial 160 and fiducial 174 exactly overlays fiducial 162, active region 170 of the laser die will precisely align with the core of a fiber placed in v-groove 154.
During assembly, the visual fiducial approach is implemented using a bonding system that is capable of imaging the optical device (particularly, fiducials 172 and 174) and the substrate fiducials 160 and 162. By suitable means, the optical device placement is affected such that the optical device fiducials 172 and 174 lie at a specified location relative to the substrate fiducials (typically directly atop one another). Such a process can be performed either automatically or manually. On completion of the device bond, the active region 170 will precisely align with the core of a fiber placed in the v-groove 154.
Obviously, the accuracy with which the active region 170 of the laser die mates with the core of the fiber placed in v-groove 154 depends on many factors. For instance, it depends on the accuracy of the bonding system used to place the die on the substrate. Also of great importance is the placement accuracy of the visual fiducials on the substrate wafer and on the laser die. This placement accuracy, in turn, depends upon the accuracy of the individual process masks used to define the visual fiducials and on the number of process steps used to fabricate them. As the number of process steps increases, fiducial location errors increase owing to the accumulation of errors within the individual process masks and to any alignment errors between the individual process steps.
Another well known passive alignment technique involves the use of metal pads formed on the surface of the silicon waferboard and mating metal pads formed on the surface of the optoelectronic device. Solder (or other wettable material such as brazing alloy, Babbit metals, amalgams and even certain thermoplastic polymers) is placed on the pads of either the waferboard or the optoelectronic device die or both. Then, the optoelectronic device is roughly aligned with and placed on the waferboard so that the mating pads are in contact, where this initial placement need not be particularly accurate. The solder is then reflowed (reheated to a molten state) whereby final alignment is effected by the inherent reduction in the surface tension within a liquid medium, the net effect being to draw the mating metal pads into accurate alignment with each other.
Just as discussed above in connection with the visual fiducial technique for passive alignment, the accuracy of this technique is highly dependent on the accuracy of the placement of the metalization pads, which, again, is adversely affected as the number of photolithography steps that are cumulatively relied upon in aligning the metal pads relative to the active region of the laser increases.
Accordingly, it is an object of the present invention to provide an improved technique for passively aligning an optoelectronic device with a fiber.
Accordingly, it is another object of the present invention to provide an improved technique for passively aligning a buried heterostructure laser with a fiber.
It is another object of the present invention to provide an improved method and apparatus for aligning an optoelectronic device on a mounting substrate, such as a silicon waferboard.
It is a further object of the present invention to provide an improved method for creating mechanical fiducials, visual fiducials and/or metalizations on an optoelectronic device.
The present invention relates to a technique for fabricating a mechanical alignment fiducial or visual alignment fiducial on a laser die particularly adapted for application with a laser die that is a buried structure edge emitting laser. In fabricating the device, a laser base structure consisting of (1) a buffer layer, (2) a bulk or multiple quantum well (MQW) active layer, (3) a cladding layer, and (4) a thin etch-stop alignment layer. A dielectric, such as SiO2 or Si3N4, is deposited over the wafer. A photolithography patterning step, using conventional techniques, is used to simultaneously define openings in the SiO2 or Si3N4 etching mask corresponding to the mechanical alignment fiducial or visual fiducial and to the laser waveguide mesa.
A reactive ion or other suitable etching technique is then used to etch through the etch-stop alignment layer, cladding layer, and active layer of the underlying structure to simultaneously define the mechanical or visual fiducial and the laser waveguide mesa. Photoresist is next applied and patterned such that the region in the vicinity of the alignment notch is protected with photoresist, while the active mesa region is exposed for a subsequent etching step to further define the active mesa. After the final mesa etching step, which utilizes an etchant which does not attack photoresist, the remaining photoresist is removed, leaving a structure which now incorporates a precisely located etch-stop alignment layer that can serve as a mechanical or visual fiducial, upon which additional regrowths may be performed using conventional LPE or MOCVD techniques. This incorporated layer now functions as a precise etching mask for material selective etchants, such as HCl.
Next either a conventional unmasked LPE regrowth or a conventional masked MOCVD regrowth step is performed to complete the structure. The regrowth typically consists of several appropriately doped InP layers, followed by a capping layer of, for example, InGaAs or InGaAsP. A final patterning and selective etching step is used to expose the etch-stop alignment layer bearing an accurately aligned notch at the outer edge of this layer. The precisely located surfaces of the etch-stop alignment layer can serve as either mechanical or visual fiducials. For instance, the laser can be positioned and pushed up against mating mechanical fiducials on a silicon waferboard. Alternately, the fiducials can be made to have two dimensional features and can be used as visual fiducials for use in connection with an optical alignment technique. Even further, the fiducials can be used as visual fiducials for precise alignment of metal pads that are formed on the optoelectronic device die, which metal pads will be used for later alignment of the die on a substrate using solder re-flow or other similarly principled techniques.